This invention relates to abrasive articles made using a hybrid bond material. In the context of this description, the term "abrasive article" is understood to refer to those articles more commonly described as coated abrasives and bonded abrasives.
Coated abrasives are distinguished by the use of a substrate material which is usually planar and the deposition thereon of abrasive grain bonded to the substrate by a bond material. Conventionally the bond, or a precursor thereof, is deposited on the substrate and the abrasive grain is deposited on the binder which is then cured to provide an adequate anchor for the grain. The first binder layer is referred to as the maker coat and a coat over the grain is referred to as the size coat. In an alternative configuration the abrasive grain is mixed with a binder or binder precursor and the mixture is deposited on the substrate before the binder or binder precursor is cured. The bond/abrasive layer can be deposited as a uniform layer or in a structured pattern which is either the result of the deposition process or a subsequent treatment before cure of the binder. In the latter situation the coated abrasive product is often referred to as a structured abrasive.
Bonded abrasives articles are characterized in that they comprises a three-dimensional structure in which abrasive grain is held in a matrix of a bond which is conventionally a metal, a vitreous material or an organic material. Metal bonds are generally reserved for superabrasives. Metal bonded abrasives are generally obtained in the form of thin layers of superabrasive grain brazed on to a metal wheel or surface. The present invention relates more directly to abrasive articles in which the structure is three-dimensional and the bond is a hybrid bond.
The "hybrid" bonds employed in the products according to the invention are bonds that do not fall comfortably into either vitreous or organic categories. Vitreous bonds, as the name implies are based on glassy materials that need to melt and flow to coat the abrasive grain and form bond posts linking adjacent grains before being allowed to cool to solidify and hold the structure together. Vitreous bonded materials are therefore formed at high temperatures and using protracted forming cycles. The product is however very rigid and effective particularly in precision grinding applications. Organic bonded materials are however formed at considerably lower temperatures and the bond is a polymeric material that can be shaped at relatively low temperatures and which can be caused to become rigid as a result of cross-linking. The polymer can be a thermosetting resin such as for example a phenol/formaldehyde, a urea/formaldehyde or an epoxy resin or it can be a radiation curable resin such as for example an acrylated urethane resin or acrylated epoxy resin or acrylated polyester resin or any one of the many variations on such chemical themes that produce a highly cross-linked rigid polymer upon exposure to visible light, UV light or electron beam radiation, with or without a catalyst activating or enhancing the transformation.
One useful category of hybrid polymeric materials is described in U.S. Pat. Nos. 4,349,386; 4,472,199; and 4,888,311. These describe a family of silicoaluminates, polysialates and/or (siloxo-sialate) polymers. Such polymers have the generic formula: M.sub.n [--(Si--O.sub.2 --).sub.z --Al--O.sub.2 --].sub.n.w.H.sub.2 O in which M is sodium or potassium or a mixture thereof, z is 1-3; w has a value up to 7 and n is the degree of condensation. Such polymers are now generally recognized by the trivial name "geopolymers". They are conveniently made by addition of a caustic-hydrated aluminosilicate to an alkali metal silicate solution. A minor variation on this theme produces polymers known as "geosets". These are made by the addition of a caustic solution of an alkali metal silicate to a hydrated aluminum silicate. For the sake of simplicity, both types of product will hereinafter be referred to as "geopolymers".
The use of such geopolymers in the production of bonded abrasives is recognized in EP Application 0 485 966 which also teaches that these bonds can be modified by the addition of organic polymers.
Geopolymers are characterized as "hybrid bonds" because they are not like either vitreous or organic bonds though they have some characteristics of each. They have very significant advantages over conventional vitreous bonds in the production of bonded abrasives. Of primary importance is that they form at comparatively low temperatures, (like organic bonds), that are well below the temperature at which glass is molten, and have a uniform composition. By contrast vitreous bonds must be formed at molten glass temperatures and held at such temperatures while the glass flows so as to coat the abrasive grains and form bond posts. The geopolymers however form polymeric structures with much of the hardness and strength of vitreous bonds and in this they are unlike conventional organic bonds which are much less brittle and have greater modulus values than vitreous bonds.
The use of geopolymers is therefore a very attractive alternative to conventional vitreous bonds from the point of view of their comparatively low temperature of formation. As a result of the relatively low temperature processing, many advanced technologies such as the use of active fillers which are unavailable in vitreous bonded products, can be incorporated in the bond. Added to these advantages is the higher post processing thermal stability and use temperatures by comparison with organic bonded products. The bond materials are therefore truly "hybrid" in nature.
The low processing temperature also makes possible the moderation of some of the brittleness associated with vitreous bonds by the addition of organic polymers. There is therefore the possibility of tailoring the physical properties of a bond to the needs of the product to be made.
There is however a serious problem with use of geopolymers in the production of bonded abrasive products in which the abrasive is based on alumina. This is because the bonds are formed in strongly alkaline conditions and the surface of the alumina abrasive grit is attacked by the alkali. The result is a very significantly weakened bond between the abrasive and the bond material such that in actual grinding tests the performance is quite unimpressive.
It has now been found that geopolymers can be used with alumina-based abrasives and this discovery forms the basis for this invention. This discovery opens up the possibility of low cost vitreous-bonded abrasives wherein the properties of the bonded abrasive can be adjusted by modification of the bond and wherein the bond is highly reproducible and economical to produce and use.